The "Left click on combined item" -> "Cycle through windows" option has a very simple implementation on the tweaker's side - all it does is making the taskbar think as if the control key is pressed when clicking the taskbar items. i.e. it's a built-in Windows feature, and the tweaker merely makes it simpler to use. By the way, holding the control key in this mode brings back the default behavior.

Given the above, it sounds like a Windows bug. Since it's quite specific and quite obscure, I'm reluctant to dedicate time to its investigation, to be honest. Perhaps you can find a workaround that works for you. For example, you can set up a custom mouse shortcut (such as alt+click or drag towards desktop) to show the thumbnail preview.

It's difficult for me to troubleshoot the problem since I don't have access to Outlook 365 for Enterprise. Also, it's quite surprising if 7+ Taskbar Tweaker causes such an incompatibility, it shouldn't affect other apps.

Perhaps try disabling all of the tweaker's options and see whether the issue goes away. If it does, try to find out which of the options causes the problem.

Thanks for letting me know, it should be fixed now.

You can get old versions of the tweaker here:

https://www.dropbox.com/sh/mcbychsh0xc1dvu/AAC7NwQTQ-tOTNL2BUZ_CwUua?dl=0

But even if installing v5.10 fixes your issue, I'd like to make sure the latest version works correctly for all users.

What doesn't work for you in v5.10.1? Are you getting any error message?

Fixed in v5.10.1.

Done in v5.10.1.

Should be fixed in v5.10.1.

Should be fixed in v5.10.1.

Should be fixed in v5.10.1.